To practise soft prolongation, begin with the short fathah and glide gently into waw or ya. Replacing the glide with a full dammah or kasrah changes the original vowel quality. If pausing makes the following consonant still, use an allowed length while retaining that softness.
«خَوْفٌ» — fathah before soft waw, then fa.
«بَيْتٌ» — fathah before soft ya, then ta.
«قِيلَ» — by contrast, ya follows kasrah and is a madd ya, not a lin ya.
The written form of waw or ya is not enough: the preceding vowel and the presence of a pause determine the case. Comparing a word in joining and pausing makes the difference more audible. Guided listening can also catch the common mistake of turning the soft glide into an entirely different vowel.